The present invention relates to a control system for a power converter for driving induction motors which a primary current frequency driving the induction motors is controlled for the purpose of controlling the speed of the motors, and more particularly to such a control system having a function of separately controlling a current component of the induction motor primary current contributing to a magnetic flux generation and a current component contributing to torque generation. The former current component is called an exciting current component or an exciting current and the latter current component is called a torque current component or a torque current. Such separate control of the exciting current and the torque current is called a vector control and is known to provide a control performance similar to the control over a separately excited DC motor. In other words, by separate control of the exciting current, stable field control is possible while by separate control of the torque current, a speed response and a speed control accuracy are improved.
But where the system is used for driving a plurality of motors, there occurs a problem of over-voltage and over-excitation when the number of operating motors is reduced. There has not been a satisfactory solution to this problem.